1. Field of the Disclosure
This invention is related to arrangements of transistors within integrated circuits. In particular, the invention is related to the design of digital-to-analog converters that occupy less area on a semiconductor substrate than conventional arrangements.
2. Background
Many integrated circuits perform functions that require the manipulation of both digital and analog signals. For example, the functions of counting, remembering, and timing are often best accomplished with digital signals, whereas sensing, amplifying, and controlling often favor analog signals. In a typical application, an integrated circuit may receive an analog input, convert the input to digital form for processing, and then convert the digital result to an analog output.
The conversion from a digital signal to an analog signal requires a digital-to-analog converter (DAC). A typical DAC in an integrated circuit assigns binary-weighted currents to each digit of a binary number. The individual currents are summed to create an analog current that is proportional to the value of the binary number.
Each binary weighted current is produced by a current source that includes one or more transistors. In conventional designs, the number of transistors in each current source increases exponentially as a power of two as the number of digits increases. Therefore, a DAC for ten bits will typically require more than 1000 transistors for the most basic design, and more than 2000 transistors for applications that require tighter tolerances. A basic DAC for twelve bits will typically require more than 4000 transistors.
A major contributor to the cost of an integrated circuit is the size of the semiconductor die that contains the components. Each transistor occupies a space on the die that cannot be smaller than a minimum area that is a characteristic of the particular process that is used to fabricate the integrated circuit.
An integrated circuit that relies heavily on digital signal processing may contain hundreds of thousands of transistors, so a DAC that uses only a few thousand transistors will not contribute significantly to the size and the cost of the device. However, a small integrated circuit for an application that favors analog signal processing, such as for example a controller for a power supply, cannot include such a large DAC without incurring a significant penalty from an increase in the size and cost. Therefore, it is advantageous to have a compact DAC structure for applications that call for small, low-cost integrated circuits.